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Forged pistons - What model are you running


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I’m in the process of figuring out what’s going back in the engine during the rebuild. I may buy a shortblock and just pull the stock pistons to replace with Manleys. I’m aware of the different metals and that these are more prone to piston slap. I plan to daily drive the car.

Can anyone vouch for a quiet well made piston they’ve been running?

What compression ratios are you seeing with said piston?

What model was your piston originally intended for?

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I would think you would want to do the rods as well, since those are the next most likely failure. Assuming you have taken care of deficiencies in the valve train as well.

 

I am interested on hearing opinions for pistons as well, although I'm more likely to have a shop build my engine. Maybe...

 

Sent from my SM-G965U using Tapatalk

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I would think you would want to do the rods as well, since those are the next most likely failure. Assuming you have taken care of deficiencies in the valve train as well.

 

I am interested on hearing opinions for pistons as well, although I'm more likely to have a shop build my engine. Maybe...

 

Sent from my SM-G965U using Tapatalk

 

From what I’ve read, the stock connecting rods are forged. I’m thinking my best option right now is to buy a new shortblock assembled from Subaru, and swap the pistons out for forged.

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I did Manley Piston and Rods when I had it rebuilt at 155k

 

The tuner also mentioned Manley. I’m just overly concerned with piston slap. I have a bad habit of not letting things warm up, but I do think I’ll be more careful with this kind of investment. I’ll order the shortblock and then the Manley pistons based on the bore size.

 

I’d still like to see what other people have gone with. I’ve read things that mahle is hit or miss, but I would’ve liked to go with 4032s. I almost considered sticking with stock since they’ll be brand new in the shortblock and I haven’t read of many people (fifth gens) having issues with them.

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The tuner also mentioned Manley. I’m just overly concerned with piston slap. I have a bad habit of not letting things warm up, but I do think I’ll be more careful with this kind of investment. I’ll order the shortblock and then the Manley pistons based on the bore size.

 

I’d still like to see what other people have gone with. I’ve read things that mahle is hit or miss, but I would’ve liked to go with 4032s. I almost considered sticking with stock since they’ll be brand new in the shortblock and I haven’t read of many people (fifth gens) having issues with them.

 

If you want 4032 pistons then your only choices are Mahle, Cosworth and Rallispec's Omega pistons. Mahle is top notch, it is one of the most used piston manufacturer by top motorsport teams. AFAIK Cosworth already stopped producing them for Subarus so only shops that have them in stock will be able to sell them you.

 

Piston slap on 2618 pistons shouldn't really be that big of a deal unless your engine builder doesn't know what he's doing. The bigger benefit the 4032 will have over the 2618 would be better long term wear resistance due to higher silicon content.

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Running Cosworth 4032 in my Rallispec built shortblock as I had similar piston slap concerns as you. No noise on warm up whatsoever sounds like a stock motor start up.
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JE pistons(which series I'm not positive) here with Manley turbo tuff rods. Not any louder than a typical Subaru in my mind.

Guess the answer to your compression question I would not fully know. 8.5:1 or 9.5:1 is where you'd be perhaps

Honestly just stick with another stock shortblock.

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JE pistons(which series I'm not positive) here with Manley turbo tuff rods. Not any louder than a typical Subaru in my mind.

Guess the answer to your compression question I would not fully know. 8.5:1 or 9.5:1 is where you'd be perhaps

Honestly just stick with another stock shortblock.

 

Thank you. I calculated the cr for the Manley turbo tuff pistons yesterday and it looks like I’ll be able to keep it at the stock 8.4:1. That’s going with the 17cc dish. I assumed that stock shortblock would be best, seeing as the oil pan would not bolt right up to any aftermarket block for the sti. It should be getting shipped out soon.

Edited by Brett_dub
Would not
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The 2012 Subaru service manual says our compression ratio is 9.5:1

 

I’m not sure how reliable the information is, but my Haynes manual makes mention of two different compression ratios. 2010-2011 is listed as 8.4:1 while the 2012 is listed as 9.5:1 . Now that you mention it though, im unaware of any differences between model years of the fifth gen that could cause the change. The pistons appear to be the same part number for all turbo fifth gens. Kind of strange.

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I looked up pistons in Subaru's parts catalog.

 

2010-2012 LGT have their own pistons.

2005/2006 Legacy GT/OBXT share its pistons with 04-18 STI, 05/06 Baja, FXT.

2007-2009 Legacy GT/OBXT share the piston with the similar year FXT's, 2.5L WRX

 

(The heads changed on between 06 and 07 which is probably the major reason the piston changed)

 

What does that mean, I don't know. Our pistons have a little bit of dish to them, but we also have different head (e25) than any other Subaru.

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So it looks like they must have changed the size of the chamber on the heads to get a 9.5:1 ratio for the 2012. There are different part numbers for 2010-2011 heads than for the 2012. That’s news to me.
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the 2010, 2011, and 2012 Subaru service manuals all say 9.5:1 for the LGT

 

I downloaded the 2010 & 2011 FSM and it said 8.4:1 so I was a little surprised that you wrote this. I went back look at the FSM and in the engine section 2010, 2011, 2012 the FSM all list the GT motor 9.5:1. The 2010, 2011 general section lists 8.4:1, which the 2012 general section lists 9.5:1. I suspected the 8.4:1 in the general section is a typo that was corrected in the 2012's FSM. I believe that our engine are 9.5:1

 

In looking at 2011 heads, I don't see how they would be able to change the head volume affect the CR move up 1.1 point with out significantly changing the heads as the cylinder head chamber are already on small side. I would guess the part difference between the 2010/2011 & 2012 is minor update.

Edited by dgoodhue
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I downloaded the 2010 & 2011 FSM and it said 8.4:1 so I was a little surprised that you wrote this. I went back look at the FSM and in the engine section 2010, 2011, 2012 the FSM all list the GT motor 9.5:1. The 2010, 2011 general section lists 8.4:1, which the 2012 general section lists 9.5:1. I suspected the 8.4:1 in the general section is a typo that was corrected in the 2012's FSM. I believe that our engine are 9.5:1

 

In looking at 2011 heads, I don't see how they would be able to change the head volume affect the CR move up 1.1 point with out significantly changing the heads as the cylinder head chamber are already on small side. I would guess the part difference between the 2010/2011 & 2012 is minor update.

That's weird! I only looked at the engine section. I have no idea what the difference is between the heads. The new vehicle info section certainly mentions nothing about that as a change moving from 2011->2012. I couldn't see any differences elsewhere in the service manual, either, so my I am thinking you are right - it's a typo - I would guess it changed from 2009->2010 and they didn't catch it.
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